In recent years, very extensive and elaborate refinements have been introduced to the art of multipass printing with incremental machines and methods--but in this innovative process the earlier and simpler single-pass art has been somewhat left by the wayside. Yet the benefits of single-pass printing remain important for special applications such as all-text documents--and also drafts of more complex documents. Documents printed in this way include not only letters, manuscripts, commercial advertising papers and the like, but also small items such as labels, bar codes, small receipts, credit-card charge authorizations and ATM (automatic teller machine) information slips.
Incremental printing employs individual pixel-forming devices. These take the form of thermal-inkjet nozzles, for example, or dot-matrix printing pins, or the individual heater elements of thermal-printer heads--including small units such as are used in printing the small commercial slips mentioned above.
In this document all such individual pixel-forming devices are called "printing elements" or "individual printing elements". The aggregation of printing elements is called a "printhead" or a "multiple-printing-element printhead".
One weakness of incremental printing is that the individual printing elements sometimes fail, leaving some areas of a text page or a drawing with no colorant deposited--and thereby omitting information from the image. In some cases such malfunctions can produce aesthetic defects, in other cases difficulty of reading alphanumeric characters or making out small details in a picture, and in still other cases actual loss of intelligence in numerical data, bar code lines, and the like.
Such errors may for instance include omission of crossbars in alphanumeric characters--thereby causing a numeral "8", for example, to appear as a "0". Analogously entire lines may be omitted from diagrams, e. g. floor plans--whereby for instance an entire wall between rooms may vanish. In such diagrams it is desirable to try to avoid such problems by setting the minimum line width to two pixels, but even this stratagem may be inadequate as sometimes two printing elements in a row fail.
Basically all such errors can be eliminated or concealed only by pressing into service some backup nozzle, pin, heater etc. that is functioning. Previous algorithmic efforts along these lines have resorted to multipass or at least plural-pass modes--implying that at least two different nozzles or pins must be made available to print on each pixel row of the image.
A fundamental problem with such techniques is that throughput is severely degraded by switching to plural-pass modes. In particular if a printing system detects just one single printing element malfunctioning, in accordance with the described technique the deficit can be overcome only by changing to a printing mode of at least two lasses.
In other words, throughput in terms of area covered per scan (i. e., per reciprocation of the scanning carriage) must be cut in half to accommodate a loss of only one printing element, even though for example an inkjet pen may have, say, two hundred to twelve hundred nozzles. Plainly a fifty-percent throughput loss is disproportionate to a 1/12 of one percent or 1/2 of one percent loss in nozzle complement.
Even dot-matrix machines may typically have more than twenty-four pins. In this case if one pin fails, the result is a fifty-percent throughput loss for approximately a four-percent pin failure--still severely out of proportion.
Conclusion--The established techniques accordingly have continued to impede achievement of high-throughput, reasonably high-quality text or draft printing. Thus important aspects of the technology used in the field of the invention remain amenable to useful refinement.